ENGLISH ABSTRACT: Biochar has been labelled to be a key factor in the global carbon mitigation act and has
been described as the modern day equivalent (terra nova) to the terra preta dark earth
soils of the Brazilian Amazon. Globally biochar has been evaluated as a means to
improve soil fertility and to mitigate greenhouse gases (GHGs). Little research has
however been published on the effects of biochar incorporation on soil physical
properties.
The objective of this study was to evaluate the effect of pine sawmill waste derived
biochar (locally-produced via slow pyrolysis – 450°C) on selected soil physical
properties, soil-water dynamics and crop production and- performance, when amended
to a Kroonstad (Kd 1000 – Morgendal) soil form. This soil form is commonly found in the
Western Cape area (South Africa) and can be classified as having low agricultural
suitability for perennial- and annual crop species.
Two pot trials were carried out in an atmospheric controlled greenhouse, where winter
wheat and green beans respectively were planted, with five different application levels
of biochar (0t/ha, 1t/ha, 10t/ha, 50t/ha and 200t/ha). Soil physical properties namely,
water-stable aggregates, bulk density and water-retention capacity along with
physiochemical characterisation of the sandy soil and biochar was determined. The
water-use was monitored throughout the trials (evapotranspiration, volumetric water
content and biomass water use efficiency, BWUE). The above- and below ground
(specific leaf traits for the green bean and the root structural development for the winter
wheat) biomass was collected and analysed at harvest.
There was significantly higher volumetric water content measured for the 50t/ha and
200t/ha biochar treatments. This effect can be ascribed due to a change in the soil’s
tortuosity and porosity where more meso- and micro-pores were present as the biochar
rate increased. The same results were evident when a water-retention curve was
established in vitro by means of the sandbox method. The bulk densities were only
significantly lower for the 200t/ha biochar treatments.
The wheat root systems differed greatly among the fertilised biochar treatments: the
50t/ha and 200t/ha treatments had a more complex fibrous root system (more extensive
branching and thinner roots) than 0t/ha, 1t/ha and 10t/ha application levels. This is
attributed to the increased water-holding capacity along with a reduction of N- and P
availability with increasing addition of biochar. Several leaf traits were measured for the
green bean crops; however the leaf nitrogen- and carbon content, chlorophyll content
index (CCI) and carbon isotope fractionation yielded the most interesting findings.
Concerning the fertilised biochar treatments, there was established that the 10t/ha
treatments had the highest leaf nitrogen- and carbon content. The leaf chlorophyll
content did not differ significantly between the fertilised biochar treatments; however a
very interesting observation was evident regarding the measured leaf CCI for the
unfertilised treatments. A decreasing trend and lower leaf CCI was measured as the
biochar application levels increased. This effect was ascribed to be due to a decrease in
N uptake by the plants as the biochar application increased, the C/N ratio also
increased, and this leading to N immobilisation. The lowest leaf carbon isotope
fractionation was measured for the 10t/ha fertilised treatments and is inversely
correlated with BWUE and therefore endorses the conclusion that the 10t/ha biochar
application had a positive effect on the long term water use efficiency for the green bean
plants.
Biochar promoted aggregation in the sand-rhizosphere interface for winter wheat,
increased water-holding capacity and enhanced crop performance for green beans.
The findings reported here provide new information on the effect of biochar on the
structural development of sandy soil, combined with biochar- and root growth effects for
winter wheat; along with detailed interpretations of specific leaf traits associated with
crop production for commercial green beans. The addition of biochar at low application
levels (approximately 1-10t/ha to 15 cm depth) increased the biomass yield and water
use efficiency of the crop species. Besides long term carbon storage, biochar can have
immediate positive effects on the physical properties of sand and plant growth.